First, change your water pump(s) impeller(s). They are only rubber and you should not even hesitate at changing them every year. Check your sea water intake and strainer(s) for blockage. Then change your thermostat ($12). The thermostat and/or its cavity are not allowing water to flow though. The stat's function is to recirculate water in the block until operating temp is reached then 'bleed' cooler sea water through and regulate the temp. If you are in salt water it eats cast iron and you can get marine growth in there as well. You did not mention if it was 'closed cooling' (sea water on one side of a heat exchanger and engine coolant/glycol on the other) or strait sea water cooling through the entire engine (least desirable as it will eat your block and manifolds/risers in about 5 years). Be inquisitive and take it apart. It is all cheaper than cooking your engine. Don't overheat it or you will deserve the cost of replacing it. Nobody is going to take care of you as good as you can. Good luck
Big Dick